The present invention relates to a method and to an apparatus for the machining of material by means of a laser.
From the literature reference "Optical Studies of Pulsed-Laser Fragmentation of Biliary Calculi", Applied Physics B, Springer Verlag 1987, pages 73 to 78, there is known a method in which the output signal of a laser is directed via a laser optical system, which includes an optical wave guide, at urinary or biliary calculi so as to fragment them. In the laser optical system there is arranged a semi-transparent mirror which guides a part of the light re-emitted, reflected or backscattered by the calculus and guided back via the optical wave guide, to a detector (1) behind which an evaluation circuit in the form of a spectral analyzer is connected.
It is known from EP-A2-01 95 375 to irradiate upon the removal of deposits on tissue the machining area with a low-power pilot laser and to evaluate the re-emitted energy, for instance for three wave lengths, in order to determine whether the material impacted by the pilot laser is formed by a deposit or by tissue which is not to be impacted by a machining laser pulse. For this, however, there is required a separate pilot laser which is used, prior to triggering the machining laser pulse, to determine the material located in front of the laser optical system.
From EP-A1-0 312 650 there is known a method and/or an apparatus of the type indicated above in which only one laser is used, the pulse of which can be interrupted or at least be reduced in its power before reaching full power if it is ascertained, during the rise of the laser pulse by an evaluation of the amplitude characteristic over time of the light returning from the material prior to the dielectric breakdown, that said laser pulse impacts tissue and not material which is to be removed.
DE-A1-39 18 618 describes a similar method and apparatus for the machining of material by means of a laser in which there is measured, in the period between the start of the laser pulse and the earliest possible occurrence of the dielectric breakdown, the intensity of the light re-emitted by the material in at least two predetermined spectral ranges and a quotient of the measured values of the light energy in these predetermined spectral ranges is formed and evaluated and used for the determination of the material impacted by the laser pulse. This method is possible in those cases where corresponding significant differences are present in the spectral characteristic of the light re-emitted by different materials. This, however, is not the case in some applications, for instance in the field of angioplasty. Thus, for instance, normal healthy intima and calcified plaque do not show any significant differences in the fluorescence characteristics upon excitation with a pulsed laser of a wave length of 375 nm and can therefore not be distinguished from each other by this evaluation method, while, for instance, fibrous fatty plaque can be distinguished.